SPORULATION OF PLANT PATHOGENS 377 



of reproduction which occurs. The explanation proposed by Cantino 

 for the effect of bicarbonate is that the ion in some way interferes with 

 the oxidative decarboxylation of a -ketoglutaric acid. Concurrently, 

 several other enzyme activities of the plant disappear and a new poly- 

 phenol oxidase becomes evident. The upshot of these changes is a 

 reorientation of metabolism resulting in the formation of the resistant 

 sporangium; this involves necessarily the synthesis of the chitin, me- 

 lanin, and y-carotene characteristic of the resistant sporangium. 



These conclusions have been buttressed by study of a mutant strain 

 of B. emersonii which does not, even with bicarbonate, form resistant 

 sporangia. Crude cell-free extracts from the mutant strain are devoid 

 of aconitase and of the enzyme system oxidizing a-ketoglutarate via the 

 cytochromes (65). Precisely these enzymes are postulated to be re- 

 quired in the normal strain for the expression of the bicarbonate effect. 



The rather elaborate metabolic hypotheses which have been erected 

 in the course of this study of reproduction in Blastocladiella are some- 

 what provisional and speculative; further investigation is clearly neces- 

 sary. But, however the hypothesis may have to be modified later, it 

 should be recognized as a bold and imaginative attempt to correlate 

 biochemical events and patterns of morphogenesis. 



12. THE SPORULATION OF PLANT PATHOGENS 



Reproductive processes in fungi pathogenic to or deriving their food 

 from higher plants are difficult to study but pose at least two interesting 

 questions: the influence of the host on sporulation in general, and spe- 

 cific relations between the host and particular stages in the life cycle of 

 the pathogen. 



It is a commonplace that plant pathogens often lose their capacity to 

 sporulate during continuous artificial culture. Of course, this may 

 only be an example of the very common phenomenon, not at all re- 

 stricted to plant pathogens, of the selection in culture of non-sporulat- 

 ing variants (77, 127, 128, 138, 207, 213, 223, 244, 310, 311). However, 

 there is still the question of how contact with the host — often only in 

 a single passage — acts to select for restoration of reproductive capacity. 



It seems probable that specific features of the food plant promote 

 sporulation over and above any selective process. We have already 

 noted the important observation that the host tissue replaces a 

 light requirement for the sporulation of HelmintJiosporium gramin- 

 eum. Experiments with autoclaved plant extracts are of little real 

 help in working out the problem, since labile materials are probably 

 involved. Cold sterilization of natural substrates by, for example, 



